Chemical ionization involves the transfer of charged species from reagent ions to analyte molecules to produce analyte ions that can be subsequently mass analysed. The charged species most commonly formed in positive ion mode is the adduct between the analyte molecule and positive hydrogen ions (H+).
Chemical ionization conducted at atmospheric pressure is known as Atmospheric Pressure Chemical Ionization (“APCI”). A sample containing analyte material is typically delivered to an Atmospheric Pressure Chemical Ionization ion source as a solution. The solution containing the analyte is then sprayed into a heated tube through which a nebulising gas is also directed. The nebulising gas causes the sprayed solution to be nebulised into fine droplets which then impact the inner wall of the heated tube and are converted into the gas phase. As the solution is converted into the gas phase the analyte molecules become desolvated. Hot gas comprising mobile phase solvents, microdroplets and desolvated analyte molecules then exit the heated tube and expand towards a corona needle. The analyte molecules are then ionised by chemical ionization with reagent ions produced by a corona discharge in the presence of a reagent gas. In particular, analyte molecules are ionised by gas phase ion-molecule reactions between reagent ions and analyte molecules.
In this conventional arrangement, analytes that exit the heated tube in the form of neutral gaseous molecules, ions or charged micro-droplets directly pass the corona needle prior to entering the vacuum section of a mass spectrometer via an ion sampling orifice. Only a relatively small proportion of the analyte ions formed at atmospheric pressure are actually drawn through a small aperture into the vacuum system of the mass spectrometer for subsequent mass analysis
Reagent ions which transfer charged species to the analyte molecules to form analyte ions are produced as a result of a corona discharge in solvent vapour. The corona discharge is generated by applying a high voltage (e.g. 5 kV) to the tip of a sharp corona needle or pin.
Analyte molecules are ionised by gas phase ion-molecule reactions with reagent ions in the region between the corona tip and the ion sampling orifice. Analyte ions are therefore generated in the region of the corona discharge since this is also where the reagent ions are formed.
The majority of the gas exits the ion source via an exhaust port whilst a small proportion of the gas and analyte ions will be drawn through the ion sampling orifice into the vacuum system of the mass spectrometer for subsequent mass analysis.
Analyte samples which are low to moderately polar when analysed by Atmospheric Pressure Chemical Ionisation typically exhibit an increase in ion signal intensity as the voltage or current applied to the corona needle is increased. In contrast, highly polar or ionic analytes typically exhibit a decrease in ion signal intensity as the voltage or current applied to the corona needle is increased. Therefore, in order to achieve a sufficiently high ion signal intensity for highly polar or ionic analytes these analytes are conventionally generated using an ion source other than an Atmospheric Pressure Chemical Ionisation ion source, such as, for example, an Electrospray Ionisation (“ESI”) ion source.
It is believed that in Atmospheric Pressure Chemical Ionisation ion sources highly polar or ionic analytes emerge from the outlet of the heated tube in the form of ions or charged micro-droplets before the analytes have had an opportunity to interact with reagent ions. As the corona needle is maintained at a relatively high positive potential (for positive ion analysis) an electric field is generated in the region of the corona needle. The electric field generated by the corona needle will tend to retard and disperse the already positively charged analyte ions or micro-droplets which exit the heated tube causing the analyte ions or charged analyte micro-droplets to become defocussed in the region of the ion sampling orifice. Accordingly, if the voltage or current applied to the corona needle is further increased then the positive analyte ions or micro-droplets will simply be retarded and dispersed to an even greater extent and hence even fewer analyte ions will pass through the ion sampling orifice into the main body of the mass spectrometer for subsequent mass analysis and detection. Accordingly, the ion signal intensity for highly polar or ionic analytes is significantly decreased as the corona current is increased.
It follows that the ion signal intensity for highly polar or ionic analytes is optimized when a relatively low current or voltage is applied to the corona needle. In contrast, the ion signal intensity for low to moderately polar analytes is optimized when a relatively high current or voltage is applied to the corona needle. This is because when a higher current or voltage is applied to the corona needle a higher number of reagent ions are generated in the region of the corona needle. The increased number of reagent ions interact with the analyte molecules and generate a higher number of analyte ions. As low to moderately polar analytes do not generally become charged before they exit the heated tube and approach the corona needle, the low to moderately polar analyte molecules are not retarded and dispersed by the electric field generated by the corona needle. Accordingly, as the current or voltage applied to the corona needle is increased a higher number of analyte ions are generated (due to the increased number of reagent ions produced) and these analyte ions pass through the ion sampling orifice for subsequent mass analysis and hence a greater ion signal intensity is detected.
It will be appreciated, therefore, that in order to analyse samples containing a mixture of both low to moderately polar analytes and also highly polar or ionic analytes using a conventional Atmospheric Pressure Chemical Ionisation ion source, that it is necessary to execute multiple sequential experimental runs in which different voltages or currents are applied to the corona needle of the ion source (e.g. a relatively low corona current is set in a first experimental run so that ionisation is optimised for highly polar analytes and a relatively high corona current is set in a second experimental run so that ionisation is optimised for low to moderately polar analytes). Executing multiple experimental runs whilst applying different voltages or currents to the corona needle yields multiple sets of data which together provide a relatively high ion signal intensity for each analyte in the sample irrespective of the polarities or ionic nature of the analytes in the sample. However, the requirement to repeat the data acquisition process whilst applying different voltages or currents to the corona needle increases both the sample analysis time and the sample consumption volume. This can be a particular problem especially when only very small amounts of sample are available for analysis and also when the sample supplied to the ion source is dynamically changing in a short period of time, for example in chromatography applications.
It is therefore desired to provide an improved ion source.